Perchlorate is an anion that exists in the environment as a part of other compounds, paired with cations such as in ammonium, potassium, or sodium perchlorates. Ammonium perchlorate, which comprises the bulk of manufactured perchlorate, is used as an oxygen-adding component in solid fuel propellant for rockets, missiles, and fireworks. Because of its limited shelf-life, inventories of ammonium perchlorate must be periodically replaced. Thus, large volumes of the compound have been disposed of since the 1950's.
Recent studies have shown that perchlorate can affect the thyroid gland, and, therefore, affect metabolism, growth, and development. Due to these studies, the Federal Environmental Protection Agency (EPA) has placed perchlorate on its Contaminant Candidate List for further study and potential regulatory action. Both California and Nevada have set action levels of eighteen parts per billion for perchlorate under their drinking water regulations. In a report published in January of 2002, the EPA have set a proposed action limit for perchlorate at 1.5 parts per billion. Because current regulatory actions regarding perchlorate have begun and future regulatory actions regarding perchlorate appear certain, regulatory agencies have focused upon testing methods for perchlorate.
Perchlorate is currently detected and quantified using ion chromatography. The two steps to this process are: (1) extraction and separation of perchlorate from all other species in a sample, and (2) measurement of the separated perchlorate against suitable standards. There are problems associated with obtaining low levels of perchlorate in certain types of samples using the standard ion chromatography configuration. Interferences caused by a large amount of anionics other than perchlorate within a sample can lead to false positives and/or reduced detection limits. The federal EPA method suggests that pretreating the sample through dilution can potentially assist with these problems, but the dilution may cause a reduction of the concentration of the target analyte to the point where it becomes undetectable. These problems are especially problematic in samples obtained from sources that contain extremely complex matrices of components, such as seawater. In practice, this current detection method is capable of relatively low detection levels of perchlorate in samples with low levels of ionic interferences. However, prior to the present invention, no known analysis method or device can meet the proposed action limit being considered by the EPA mentioned above.
The method of the present invention overcomes the limitations of the ion chromatography method by use of reverse phase liquid chromatography, coupled with a mass spectrometric detection method.
Liquid chromatography is a technique for separating the individual compounds that exist in a subject sample. In employing the technique, the subject sample is carried in a liquid, called a mobile phase. The mobile phase carrying the subject sample is caused to migrate through a media, called a stationary phase. Different compounds will have differing rates of migration through the media, which effects the separation of the components in the subject sample. Liquid chromatography is commonly performed with reusable columns or with disposable cartridges, both of which are usually cylindrical, in which the media bed is bounded axially by porous plates, or plates containing defined flow paths, through which the mobile phase will flow. (See U.S. Pat. No. 4,250,035 to McDonald et al. and U.S. Pat. No. 5,601,708 to Leavesley)
A significant element in the LC system is the column. A typical column usually consists of a piece of steel tubing which has been packed with a “packing” material. The “packing” consists of either particulate material “packed” inside the column, or a monolithic porous phase. It usually consists of silica-or polymer-based particles, which are often chemically bonded with a chemical functionality. When the sample is carried through the column (along with the mobile phase), the various components (solutes) in the sample migrate through the packing within the column at different rates. Because of the different rates of movement, the components gradually separate as they move through the column. Differential migration is affected by factors such as the composition of the mobile phase, the composition of the stationary phase (i.e., the material with which the column is “packed”), and the temperature at which the separation takes place. Thus, such factors will influence the separation of the sample's various components. A more detailed description of the separation process can be found, among other places, in Chapters 2 and 5 of Introduction to Modern Liquid Chromatography (2d ed. 1979) by L. R. Snyder and J. J. Kirkland, which chapters are incorporated by reference herein.